The present invention relates to a tristate output gate structure particularly for CMOS integrated circuits.
The output gates of integrated circuits are currently manufactured with various methods which are able to avoid bus-conflict, which can occur on a connecting bus or line shared by various outputs of different integrated circuits, or by various outputs of a single complex integrated circuit.
In particular, and as described in greater detail hereinafter, tristate CMOS gates are very suitable for this purpose. Said gates currently have an input the effect whereof on the output is activated by means of a pair of enable channels arranged in input to said gate. Said channels are logically opposite to one another, and each enables a complementary-technology transistor. The output is connected between said two transistors.
The use of two inable signals is redundant and is also the source of manufacturing problems ahead of the output gate. Furthermore, in order to have outpus with a logic level which corresponds to the input, pairs of transistors made in the same technology and arranged in series are usually provided.
The problem arises, in this case, from the reaction times of the different pairs, one with P-channel transistors, the other with N-channel transistors.
A P-channel transistor is intrinsically slower, due to its execution, than an N-channel transistor, dimensions being equal.
Thus, in order to make their speeds equal, it is necessary to manufacture P-channel tansistors which are larger than N-channel transistors. However, this causes a corresponding increase in parasitic capacitors, which reach values higher than that of the N-channel transistor. This, combined with the intrinsic electric resistance of the P-channel transistor, entails a charge and discharge time constant, given by the product of the resistance and the value of the parasitic capacitor, which is greater than that of an N-channel transistor.
This effect is aggravated by the series arrangement of two P-channel transistors, since the time constants are not added but multiplied by one another, causing a quadratic increase in said time constant.